One repays a teacher badly by remaining only a pupil, according to philosopher Friedrich Nietzsche. But if the pupil plucks a plum that the teacher unintentionally offers and then capitalizes on it for her own gain, has the teacher been honored or betrayed?

That question intrigues actor Sarah Day, who portrays “teacher” Ruth Steiner in Donald Margulies’ “Collected Stories,” playing now through Dec. 16 at the Milwaukee Chamber Theatre. It also raises the question of what separates public from private information in the age of social media. (The joint production with Forward Theater moves to Madison on Jan. 17.)

Day appears opposite Milwaukee actor Laura Frye, who plays Lisa – the student who sees Ruth first as a mentor, then a friend and, eventually, a competitor for the attention of New York’s literary world. Lisa’s unapproved usage of private information about Ruth’s one-time love affair with poet Delmore Schwartz leads to Lisa’s fame and the friendship’s undoing.

The relationship between a mentor and her charge is something Day understands. She sees aspects of herself in both Ruth and Lisa.

“When you ask an actor about a character she is portraying, you will almost always find a cheerleader for that character,” says Day, 54, an American Players Theatre company member. “I think there is a great nobility in Ruth, but there is also a danger that she may become a hermit and a hoarder as she cocoons herself into her own world.”

As Ruth counsels Lisa and criticizes her tendency to “up-talk” – that is, to end every sentence with a question mark – the younger woman challenges her mentor’s perceptions, drawing her out of herself and into a position of trust and friendship. The revelation of the love affair, which Day says is very hurtful to Ruth, signals the betrayal of the growing trust, but also becomes a jumping off point for Lisa’s own career.

Day is looking forward to playing the character in a show with an extended run in two different cities.

“It’s fun to be able to get a little deeper into the character,” says Day, who also appeared in last season’s joint Chamber/Forward production of Gwendolyn Rice’s “A Thousand Words.”

Much of the discussion during talkback sessions after the Milwaukee performances focused on whether Lisa was wrong to go public with the information of the love affair, even though it was veiled in the character’s short stories.

The debate more or less divided along age lines, with younger audience members seeing nothing wrong in capitalizing on information that was freely offered. Older audience members, including Day herself, tend to see the information as something shared among friends and not meant for public broadcast.

“It gets me to thinking about who has the right to other people’s information,” says Day. “It has to do with the context of social networking, something of which I am not generally part.”

The joint production of “Collected Stories” continues its run at Madison’s Overture Center next month.